Generate grid overlays on reference map if a grid or markers are present on unreferenced map.
Lots of maps already have a grid and location markers present. Based on suspected location of map publisher and era, a CRS might be suggested and overlayed on both or reference maps, should always be override-able by user with a known EPSG code or even custom definition. This also should be used for determining the transformation function if CRS type is known.
Max Friedrich Hartmann commented
Especially with the Rumsey maps there are whole series of complete map collections with a known CRS and scale, assuming no distortion in scan two control points would be enough for these to compensate for rotation.
The Control points coordinates should be than snapped to a grid present in map and overlayed on reference map.